A metallic paint and metallic ink have recently been used in various fields of automobile finishes, electric appliances, buildings, stationery products, business and office machines, communication apparatuses, cosmetics and the like, due to specific designability thereof.
A film formed with a metallic paint reflects external incident light with a flaky metallic pigment contained in the film, to exhibit a glittery design. The reflection of light presents a unique appearance excellent in designability along with each color tone of the film.
Aluminum is excellent in metallic luster, low-priced and easy to handle due to small specific gravity, and hence a metallic pigment mainly containing aluminum flakes (simply referred to as “aluminum pigment” in this specification) is applied to a metallic design for automobile paint finish, plastic paint finish, printing ink or a resin molding in general.
In the field of paints, on the other hand, a water-based paint containing no organic solvent is increasingly used as a resource-saving and non-polluting countermeasure. Therefore, a demand for an aluminum pigment having excellent water dispersibility preferably compoundable with the water-based paint, is also enlarged.
In order to satisfy this demand, an aluminum pigment prepared by treating the surfaces of aluminum flakes with saturated or unsaturated fatty acid such as stearic acid or oleic acid and a derivative thereof and adding a surface active agent thereto for providing water dispersibility has been generally provided as an aluminum pigment employed for a water-based paint.
However, this water-dispersive aluminum pigment disadvantageously exhibits low storage stability upon blending into a water-based paint and low weather resistance after application. In other words, a water-based paint blended with the water-dispersive aluminum pigment causes such a problem that dispersibility of the aluminum pigment is reduced during storage to result in seeding and remarkably deteriorating the properties of the paint or the aluminum pigment reacts with water to generate a large quantity of hydrogen gas exploding a container. When the water-based paint is applied to a portion such as the finishes of an automobile or a building exposed to severe environment, further, the designability is disadvantageously deteriorated after the application.
In general, aluminum having a natural protective oxide layer on the surface thereof is stable in the atmosphere containing oxygen and exhibits excellent corrosion resistance against an electrolyte contained in neutral water. However, the natural protective oxide layer of aluminum may not exhibit sufficient corrosion resistance against an acidic or alkaline aqueous solution but may be dissolved. Particularly in recent years, a metallic paint containing an aluminum pigment has been frequently exposed to environment having severe acid conditions due to acid rain or the like, and it is difficult to sufficiently protect an aluminum pigment from corrosion with only the natural protective oxide layer.
While a large number of techniques have been developed in order to solve these problems, techniques disclosed in Japanese Patent Publication No. 1-54386, Japanese Patent Laying-Open No. 59-74201, Japanese Patent Laying-Open No. 55-158202, Japanese Patent Laying-Open No. 4-318181, U.S. Pat. No. 5,296,032, Japanese Patent Laying-Open No. 6-57171 and the like can be listed as relatively practical recent well-known techniques.
For example, Japanese Patent Publication No. 1-54386 discloses a method of treating an aluminum pigment with chromic acid. While chemical stability of the aluminum pigment is improved according to this method, the reactivity of a treatment solution is too strong to treat a fine aluminum pigment or a problem arises in labor sanitation or environment due to employment of a hexavalent chromium compound.
Japanese Patent Laying-Open No. 59-74201 discloses a method of treating an aluminum pigment with ammonium vanadate. This treatment is not yet put into practice due to large change of the color tone of the aluminum pigment resulting from the treatment.
Further, Japanese Patent Laying-Open No. 55-158202 discloses a method of covering an aluminum pigment with acidic phosphoric ester. While an excellent color tone and excellent chemical stability are attained according to this method, a treating agent exerts unpreferable influence on adhesion between the aluminum pigment and resin when preparing a two-coat one-bake or two-coat two-bake film to result in a remarkable problem of interlayer peeling between a base metallic film layer and a top coat layer.
Japanese Patent Laying-Open No. 4-318181 discloses a method of treating an aluminum pigment with a treatment solution containing an oxidizer such as molybdic acid, phosphate anions and alkaline earth metal ions. While an aluminum pigment having excellent chemical stability can be prepared by this method, the phosphate anions or the alkaline earth metal ions contained in the treatment solution tend to deteriorate the physical properties of the film such as moisture resistance.
While U.S. Pat. No. 5,296,032 discloses a method of treating an aluminum pigment with heteropolyanions of phosphomolybdic acid or the like, no sufficient chemical stability of the aluminum pigment can be attained through this treatment.
Further, Japanese Patent Laying-Open No. 6-57171 discloses a method of treating an aluminum pigment with ammonium molybdate and thereafter further stabilizing the aluminum pigment by adding molybdate or the like. While it is possible to obtain an aluminum pigment having excellent chemical stability according to this method with no problem as to film physical properties, manufacturing steps are disadvantageously complicated.
U.S. Pat. No. 2,885,366 discloses a method of covering various core materials with amorphous silica layers, and also describes Example employing aluminum powder as a core material. However, this silica covering is performed in an aqueous solution having such strong alkalinity that the aluminum powder may react to generate a large quantity of hydrogen gas during a treatment step, to result in an extreme risk. This patent also discloses a method of providing an intermediate layer of basic salt of aluminum or chromium on the core material in order to efficiently cover the same with the amorphous silica layer, it has been difficult to homogeneously cover an aluminum pigment.
While Japanese Patent Laying-Open No. 7-3185 discloses a method of manufacturing an aluminum pigment having excellent corrosion resistance by covering the surface of an aluminum pigment with siloxane and further covering the same with synthetic resin, the thickness of the aluminum pigment having the siloxane coat covered with synthetic resin is increased to disadvantageously deteriorate the appearance/gloss or reduce opacity by protrusion from the film.
While provision of an aluminum pigment having excellent dispersibility and stability, neither generating hydrogen gas nor agglomerating during storage and providing excellent designability for the appearance of a film is strongly demanded as hereinabove described, no well-known technique can provide an aluminum pigment sufficiently requiring this requirement.